The present invention relates generally to exposure apparatuses, and more particularly to a reflection type demagnification projection optical system, an exposure apparatus, and a device fabricating method. The reflection type demagnification projection optical system uses ultraviolet (xe2x80x9cUVxe2x80x9d) and extreme ultraviolet (xe2x80x9cEUVxe2x80x9d) light to project and expose an object such as a single crystal substrate for a semiconductor wafer, and a glass plate for a liquid crystal display (LCD).
Along with the recent demand on smaller and lower profile electronic devices, minute semiconductor devices to be mounted onto these electronic devices have been increasingly demanded. For example, a design rule for a mask pattern requires that an image with a size of a line and space (LandS) of less than 0.1 xcexcm be extensively formed, and predictably, it will further move to a formation of circuit patterns of less than 80 nm in the future. LandS denotes an image projected to a wafer in exposure with equal line and space widths, and serves as an index of exposure resolution.
A projection exposure apparatus, which is a typical exposure apparatus for fabricating semiconductor devices, includes a projection optical system that projects and exposes a pattern drawn on a mask or a reticle (which are used interchangeably in the present application) onto a wafer. Resolution R of a projection exposure apparatus (a minimum size which enables a precise transfer of an image) can be given by using a light-source wavelength xcex and the numerical aperture (NA) of the projection optical system as in the following equation:                     R        =                              k            1                    xc3x97                      λ            NA                                              (        1        )            
Therefore, the shorter the wavelength becomes, and the higher the NA increases, the better the resolution becomes. In the meantime, a focusing range that maintains a desired image-forming performance is called a depth of focus (xe2x80x9cDOFxe2x80x9d), and the DOF is given in the following equation.                     DOF        =                              k            2                    xc3x97                      λ                          NA              2                                                          (        2        )            
Therefore, the shorter the wavelength becomes, and the higher the NA increases, the smaller the DOF becomes. A small DOF would make focus adjustment difficult, as well as requiring higher flatness for a substrate and more precise focusing accuracy, and thus, a large DOF is basically desirable.
It can be understood from the equations 1 and 2 that a shortened wavelength is more desirable than an increased NA. In recent years, F2 excimer laser (with a wavelength of 157 nm), UV and EUV (extreme ultraviolet) light have been put to practical use as an exposure light source.
Since a shorter wavelength of light would limit usable glass materials for transmitting the light, use of reflecting elements, i.e., mirrors for a projection optical system are advantageous instead of use of many refracting elements, i.e., lenses. Further, no glass materials are usable for UV or EUV light as exposure light, and thus a projection optical system could not include any lens. Therefore, it has been proposed to form a projection optical system only with mirrors (e.g., multilayer film mirrors). For example, a projection optical system including four mirrors is proposed in Japanese Laid-Open Patent Application No. 2000-98228, while a projection optical system including six mirrors is proposed in Japanese Laid-Open Patent Application No. 2000-100694.
However, the conventional reflection type projection optical system could not reconcile the exposure performance such as resolving power and throughput, thus disadvantageously being unable to provide high quality devices. To be more specific, if it is assumed that a reflection coefficient of one multilayer film mirror is, for example, 67%, a reflection type projection optical system including four mirrors (sometimes expressed as a four-mirror system, hereinafter) can obtain a total reflection coefficient of as much as 20%. Nevertheless, NA can only attain as much as 0.1, and does not improve resolving power. On the other hand, in using a reflection type projection optical system including six mirrors (sometimes expressed as a six-mirror system hereinafter), it may be possible to improve the resolving power since NA can be made larger by as much as 0.15, but the reflection coefficient is altogether about 9%, thus degrading throughput.
On the other hand, a reflection type projection optical system including five mirrors, i.e., an intermediate number of mirrors (sometimes expressed as a five-mirror system, hereinafter) conceivably has balanced resolving power and throughput, and U.S. Pat. No. 6,072,852 discloses a high NA, reflection type demagnification projection optical system as a five-mirror system. However, a design example of optical system in this reference sets a distance between object and image points, opposite to each other with respect to an optical axis, to be about 260 mm in a direction orthogonal to the optical axis of the optical system. Thus, disadvantageously, such a reflection type projection optical system cannot be used for an EUV lithography system that employs a 300 mm silicon wafer and a six-inch reticle, since a mask stage and a wafer stage will mechanically interfere with each other.
Accordingly, it is an exemplary object of the present invention to provide a reflection type demagnification projection optical system, an exposure apparatus, and a device fabricating method, in which the reflection type demagnification projection optical system as a five-mirror system is applicable to an EUV lithography system using a 300 mm silicon wafer and a six-inch reticle, and has balanced resolving power and throughput.
In order to accomplish the above object, the reflection type demagnification projection optical system as one aspect of the present invention has five mirrors including, in order from an object side to an image side a concave mirror (M1), a convex mirror (M2), a concave mirror (M3), a convex mirror (M4), and a concave mirror (M5), these five mirrors basically forming a coaxial system, and forming no intermediate image, and wherein an object point and an image point are respectively on opposite sides across an optical axis, and are kept 400 mmxcx9c1500 mm apart from each other with respect to a direction orthogonal to the optical axis. Such a reflection type demagnification projection optical system may widen the distance between the object point and the image point even in the five-mirror system.
At least one of these five mirrors may be an aspheric mirror. Such a five-mirror structure provides a larger object height (image height), thereby avoiding interference between both stages. Furthermore, out of the five mirrors, the fourth convex mirror (M4) may be located at an aperture stop. An adequately set position and shape of the fifth concave mirror (M5) relative to the convex mirror (M4) would create a telecentric optical system on the image side. The five mirrors may multilayer film mirrors for reflecting EUV light, thus being able to efficiently reflect a ray having a wavelength of 20 nm or less. The reflection type demagnification projection optical system may arrange a reflective mask at the object side.
An exposure apparatus as another aspect of the present invention includes the above projection optical system, a first stage for holding the mask so that a mask pattern may be located on the object side, a second stage for holding a substrate so that a photosensitive layer may be located at the image side, an illumination optical system that illuminates the above mask by using arc-shaped EUV light corresponding to an arc-shaped field of the projection optical system, and a mechanism that synchronously scans the first and second stages. Such an exposure apparatus includes the above reflection type demagnification optical system as one element, thereby widening an interval between the object point and the image point, and prevent the first and second stages from mechanically interfering with each other.
A device fabricating method as still another aspect of the present invention includes the steps of using the above exposure apparatus to project and expose the above plate, and performing a specified process on the object projected and exposed as above. Claims for a device fabricating method for performing operations similar to that of the above exposure apparatus cover devices as intermediate and final products. Such devices include semiconductor chips like an LSI and VLSI, CCDs, LCDs, magnetic sensors, thin film magnetic heads, and the like.
Other objects and further features of the present invention will become readily apparent from the following description of the preferred embodiments with reference to accompanying drawings.